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Ultrafast Laser Tech Can Spot Atomic-Level Defects in Semiconductors

An illustration showing a red oscillating laser beam shooting towards an STM tip and a material surface.
Researchers combined terahertz laser light, shown as a red wavy arrow, with a scanning tunneling microscope (STM) tip — the dark pyramid shape exchanging a red electron with a sample shown with a blue surface. Credit: Eve Ammerman / Michigan State University.
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As electronics get smaller and more powerful, scientists must continually create innovative new ways for testing electronic components with even greater levels of precision.

By combining a high-resolution microscopy technique with the use of ultrafast lasers, physicists at Michigan State University (MSU) have developed a new method that is precise enough to spot single-atom “defects” on a semiconductor surface. The new analysis technique is described in Nature Photonics.

Nanoscale architecture is crucial to modern semiconductors

While the term “defects” might sound like these misfit atoms are a negative thing to be avoided, defects are critically important to the performance of semiconductors and are usually added to these materials intentionally. Defects play a significant role in electron motion through the semiconductor material, and so being able to precisely pinpoint where these defects are could help scientists to better understand the behavior of a given semiconductor.

“This is particularly relevant for components with nanoscale structures,” said study author Tyler Cocker, an assistant professor in the MSU Department of Physics and Astronomy and the Jerry Cowen Endowed Chair in Experimental Physics.

Computer chips are a common example of semiconductor materials that routinely make use of nanoscale features. Other scientists are also in the process of developing extreme new takes on nanoscale architecture, by engineering materials that are just one atom thick.

“These nanoscopic materials are the future of semiconductors,” said Cocker, who also leads the Ultrafast Terahertz Nanoscopy Laboratory at MSU. “When you have nanoscale electronics, it’s really important to make sure that electrons can move the way you want them to.”

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The new analysis technique is easy to implement, Cocker adds, saying that the MSU team has already begun to use it for studying atomically thin graphene nanoribbons, among other materials.

“We’ve got a number of open projects where we’re using the technique with more materials and more exotic materials,” Cocker said. “We’re basically folding it into everything we do and using it as a standard technique.”

Improving upon scanning tunneling microscopy

Scanning tunneling microscopes (STMs) are already commonly used to spot single-atom defects in material surfaces.

Unlike the traditional optical microscopes found in school classrooms the world over, STMs do not rely on lenses to magnify the objects they are pointed at. Instead, STMs use a probe with an atomically sharp tip – similar to a record player’s stylus – that is brought very close to the material’s surface. When an electrical voltage is applied, electrons will begin to jump between the tip and the sample, and this can provide a wealth of atomic-level information about the sample.

But STM data has its limits. Especially in the case of gallium arsenide, an important semiconductor material that is routinely used in modern telecommunications equipment and high-efficiency solar cells, STM is not always sufficient at clearly resolving defects.

This is where the new MSU-developed method comes in – combining STM with ultrafast laser pulses shone at the STM probe tip. These pulses consist of light waves at terahertz frequencies, meaning that they oscillate a trillion times per second.

In the new Nature Photonics paper, the team describes the analysis of gallium arsenide samples that were intentionally infused with silicon defect atoms. While this type of defect has been well-studied by theoretical physicists, experimentalists have not been able to detect these single atoms directly – until now.

“The silicon atom basically looks like a deep pothole to the electrons,” Cocker said.

Combining STM with these terahertz laser pulses, the MSU team was able to clearly detect signals from the silicon defects in the semiconductor. When the STM tip with the laser applied was moved over the position of a silicon defect, the team recorded a sudden, intense signal in the STM data. When the tip was moved away from the defect, this signal disappeared.

The researchers believe the signal became detectable because the terahertz light was oscillating at the same frequency that a silicon atom should oscillate within a gallium arsenide lattice.

“Here was this defect that people have been hunting for over 40 years, and we could see it ringing like a bell,” Cocker said.

“At first, it was hard to believe because it’s so distinct,” he continued. “We had to measure it in every which way to be certain that this was real.”

While Cocker’s lab continues to use this method to analyze exotic materials, the team believes that this new approach to STM analysis could also benefit other areas of science, beyond simply detecting semiconductor defects.


Reference: Jelic V, Adams S, Hassan M, Cleland-Host K, Ammerman SE, Cocker TL. Atomic-scale terahertz time-domain spectroscopy. Nat Photon. 2024:1-7. doi: 10.1038/s41566-024-01467-2

This article is a rework of a press release issued by Michigan State University. Material has been edited for length and content.